1. Field of the Invention
The invention relates to the brazing and repair of superalloy components, and in particular to brazing compositions and methods for brazing superalloy blade and vane components used in gas turbines, with braze compositions and brazing procedures that consistently provide good brazing of test samples, some embodiments of which permit post braze welding without substantial degradation of structural properties.
2. Description of the Prior Art
Structural repair or new fabrication of nickel and cobalt based superalloy materials that are used to manufacture turbine components, such as cast turbine blades, are challenging, due in part to the metallurgic properties of the superalloy material. For example, a superalloy having more than about 6% aggregate aluminum or titanium content, such as nickel-base superalloys with low carbon content e.g., CM247, is typically more susceptible to solidification cracking when subjected to high-temperature welding than a lower aluminum-titanium content superalloy, e.g., X-750. Superalloys used in finished turbine blades are typically strengthened during post casting heat treatments, which render them difficult materials upon which to perform subsequent structural welding repairs. Currently used welding processes for superalloy fabrication or repair generally involve substantial melting of the substrate adjoining the weld preparation, and complete melting of the added welding filler material. When a blade constructed of such a material is welded with filler of the same or similar alloy, e.g., for structural repair, the blade is susceptible to solidification cracking (aka liquation cracking) within and proximate to the weld. Post weld solidification cracked superalloy vanes and blades are generally scrapped as unrepairable, after considerable time and expense was already expended to attempt to repair the blade. Given the shortcomings of superalloy structural repair welding, often the only commercially acceptable solution is to scrap damaged turbine blades that require structural repair, because past experience has shown limited success of such structural repairs. Thus repairs have been limited to those particular materials, components and types of structural damage that have in the past been proven amenable to successful repair by cosmetic welding, employing more ductile welding filler materials with reduced structural strength. Blades needing welded structural repairs with a known relatively high risk of post weld solidification cracking are generally scrapped. Providing brazing compositions and methods that can withstand post braze welding without significant solidification cracking or other degradation of structural, mechanical or other properties would permit repair and reuse of such components, an important economic benefit.
Non-structural repair or fabrication of metal components, including superalloy components, typically involves replacing damaged material (or joining two components of newly fabricated material) with mismatched alloy material of lesser structural properties, where the superior structural performance of the original substrate material is not needed in the localized region. For example, such non-structural or “cosmetic” repair may be used in order to restore the repaired component's original profile geometry. For the repair of gas turbine components, an example of cosmetic repair is the filling of surface pits, cracks or other voids on a turbine blade airfoil in order to restore its original aerodynamic profile, for cases in which the mechanical properties of the blade's localized exterior surface are not critical for the structural integrity of the entire blade. Cosmetic repair or fabrication is often achieved by using oxidation resistant weld or braze alloys of lower strength than the blade body superalloy substrate, but having higher ductility and employing a lower application temperature that does not degrade the structural or material properties of the superalloy substrate.
Diffusion brazing has been utilized to join superalloy components for repair or fabrication by interposing brazing alloy between their abutting surfaces to be joined, and heating those components in a furnace (often isolated from ambient air under vacuum or within an inert atmosphere) until the brazing alloy liquefies and diffuses within the substrates of the to-be-conjoined components. Diffusion brazing can also be used to fill surface defects, such as localized surface and/or non-structural cracks, in superalloy components by inserting brazing alloy into the defect and heating the component in a furnace to liquefy the brazing alloy and thus fill the crack. In some types of repairs a torch rather than a furnace can be used as a localized heat source to melt the brazing alloy. Braze repaired superalloy blades and vanes are typically returned to service.
In a subsequent gas turbine inspection cycle, blades or vanes that are identified as having defects in previously braze-repaired surfaces risk remelt and migration of old braze material if the component were again heated for repairs. Often for commercial cost saving reasons blades with defects in previously brazed portions are scrapped rather than risk potential repair failure attributable to remelt migration of old braze material.
Braze material with the commercial designation Mar-M-509® (A registered trademark of Martin Marietta Co. and commercially available, for example, from Praxair Surface Technologies, Inc. Indianapolis, Ind. under their designations CO-222, CO-333) is a high chrome content superalloy braze material that has commonly been used for repair of CM247 alloy turbine blade and vane components. Products with similar performance characteristics are also commercially available from Sulzer Metco as Amdry MM509 and Amdry MM509B. However, it would be desirable to utilize a braze material including CM247, so that the braze material and the component substrate have more closely matched material properties. A commercial designation for CM247 is MAR-M-247, one form of which is available from Praxair Surface Technologies under their designation NI-335-5.
Thus, a need exists in the art for a braze composition having material properties more closely matching those of CM247 superalloy components, such as gas turbine blades and vanes, that can be rewelded without melt migration from the weld zone and that resists solidification cracking at the weld interface or surrounding areas.